1. Field of the Invention
The present invention relates to a display element and a display device and a process of producing a display-related circuit in particular, and more particularly, to a display element and apparatus characterized by being formed by transferring separately-prepared thin-film-shaped circuits to and arranging them on a substrate carrying a pixel section of the display device and a process of producing the same.
2. Related Background Art
In recent years, many suggestions have been made about a display medium which is close to paper (printed matter) in form or a display medium the display content of which is electrically rewritable, under the name of digital paper, paper-like display, electronic book, or the like. More specifically, an electronic book comprised of a plurality of sheet-like page displays or the like is under study.
In such a conventional sheet-like display, wiring between a display element and control circuit is performed using an enormous number of external wires.
However, the above-described sheet-like page display is required to be as thin as possible to incorporate various electrical control circuits thereinto. This makes it necessary to mount a peripheral circuit composed of a switching circuit comprising thin-film transistors (hereinafter abbreviated as xe2x80x9cTFTxe2x80x9d) for switching pixels, a sample and hold circuit, a shift register, or the like on the periphery of pixels on a substrate.
Hitherto, there has been generally used the technique that a monocrystalline silicon substrate having a semiconductor circuit formed thereon is divided into chips and arranged on a display element substrate. These chips are called xe2x80x9cCOG (Chip On Glass)xe2x80x9d and a method called a xe2x80x9cTAB (Tape Automated Bonding)xe2x80x9d has been used to connect these circuits. However, a limit of connection pitch of TAB is said to be approximately 40 xcexcm and the method is not applicable to a display device having resolution equal to or higher than this.
On the other hand, in the field of liquid crystal display elements, it is a general practice in recent years that low temperature polycrystalline silicon transistors are used to form a peripheral circuit such as sample and hold circuit on the substrate of the display element. However, fabricating the entire control circuit for a display element using monocrystalline silicon has not been realized because of its low electron mobility. Though the term xe2x80x9clow temperaturexe2x80x9d is used, it requires a temperature on the order of 500xc2x0 C. and the substrate of the display element is required to have high heat resistance.
To compensate both defects, there is a need for a novel mount technique capable of forming a transistor circuit using monocrystalline silicon or the like on the substrate of the display element.
As a solution to this, U.S. Pat. No. 6,005,284 proposes to use a process which consists of forming a monocrystalline silicon layer on a semiconductor substrate through a porous layer, forming a desired semiconductor circuit thereon, bonding a desired support substrate to the semiconductor circuit, separating the semiconductor substrate from the porous layer using an external force (pulling force) and thereby transferring the semiconductor circuit to the support substrate.
However, according to this process, the separating step is performed with a pulling force, which is an external force, and therefore local distortion or the like is applied particularly to a part of the semiconductor circuit formed in the semiconductor layer, which may damage the semiconductor circuit or impair the device characteristic. Such a problem may occur more frequently as the transfer area increases.
As stated above, the conventional method cannot accommodate a high temperature process required in the formation of a semiconductor circuit to form a display element or a display device having a circuit which has a lower heat resistance, and the conventional method thus suffers from many problems such that it is difficult to improve the device characteristic of the semiconductor circuit or that the element characteristic is unstable when the element is formed at an insufficient temperature, thus making it also difficult to improve yield.
Thus, there is a need for a display element that reduces heat stress applied to the substrate of the display element and operates stably.
The present invention has been accomplished to solve the above-described problems and it is an object of the present invention to form a semiconductor circuit on a low heat-resistance substrate including plastics or the like, having flexibility and provide a display element and a display device, and provide a process of producing the same.
According to a first aspect of the present invention, there is provided a display device comprising an image display element section and at least one of a switching circuit section and a peripheral circuit section for driving the image display element section disposed on a first substrate,
wherein the at least one of switching circuit section and peripheral circuit section is formed by performing one or more times a forming step of forming a circuit section comprising the at least one of switching circuit section and peripheral circuit section in a circuit film disposed on a second substrate and a transferring step of transferring and disposing the circuit film having the circuit section formed therein on the first substrate.
It is preferred in the present invention that the material constituting the at least one of switching circuit section and peripheral circuit section comprises an organic semiconductor, amorphous silicon, polycrystalline silicon or monocrystalline silicon.
Further, it is preferred that the circuit film comprises a semiconductor element, a semiconductor integrated circuit, or an element of a metal/insulator/metal stack structure (MIM structure).
It is more preferred that the circuit film comprises at least one of a semiconductor element and a semiconductor integrated circuit formed in a monocrystalline silicon layer.
Moreover, it is preferred that the display device has a configuration such that the switching circuit section is disposed on a first surface of the first substrate and at least a part of the peripheral circuit section is disposed on a second surface of the first substrate.
According to a second aspect of the present invention, there is provided a process of producing the display device set forth above, comprising the following steps (1), (2), (3), (4) and (5):
(1) a step of preparing a second substrate comprising a separation layer and a semiconductor film successively stacked on a substrate;
(2) a forming step of forming at least one of switching circuit section and peripheral circuit section in the semiconductor film to form a circuit film;
(3) a separating step of separating the circuit film from the second substrate;
(4) a transferring step of transferring and disposing the circuit film onto the first substrate; and
(5) a step of forming an image display element section on the circuit film, provided that each step is performed at least once (or at least one time).
It is preferred in the present invention that the second substrate is a semiconductor substrate, and the formation of the circuit film comprises a step of forming a separation layer on the surface of the semiconductor, a step of forming a semiconductor film on the separation layer, and a step of forming the circuit section comprising at least one of a semiconductor element and a semiconductor integrated circuit in the semiconductor film.
Furthermore, it is preferred that the transferring step comprises a bonding step of bonding the circuit film to the first substrate and a separating step of separating the circuit film from the second substrate.
Further, it is especially preferred that the separation layer is a porous silicon layer.
Moreover, it is preferred that the semiconductor film comprises at least one of monocrystalline silicon and a chemical compound semiconductor.
Furthermore, the present invention includes both a process of transferring a number of the circuit films all together from the second substrate to the substrate on which the display device is formed and a process of cutting the circuit film prior to the transferring step and then transferring the circuit film divided into chips to the first substrate.
Furthermore, in the above transferring step, the circuit film may be transferred to the first substrate through a third substrate.